a. Field of the Invention
This invention relates to a numerical control apparatus and, more particularly, to a numerical control apparatus for controlling the rotational velocity of a plurality of spindles independently or simultaneously.
b. Description of the Related Art
Some machine tools such as lathes are equipped with a plurality of spindles. For example, a lathe for backface machining has two spindles in order to rotate a workpiece. Back-face machining is performed by setting a workpiece on one spindle automatically when machining using the other spindle is completed. Besides machining such as turning, lathes can apply other types of machining, such as boring and grooving. In such case, the tools themselves must be rotated, there are two tool rests, and the number of spindles is three, four or more.
In order to control the rotational velocities of a plurality of spindles, the practice in the prior art is to (i) provide a plurality of rpm command output units and a plurality of units for converting rpm commands into voltages and delivering these voltages to a spindle control amplifier, or (ii) change over one rpm command so that the command may be used to control any one of a plurality of spindles. However, these former methods require provision of a plurality of spindle control systems and therefore result in a large and costly apparatus. Such methods are not suited for the control of a number of spindles.
Further, with the latter method, an rpm command is issued for only one spindle at all times. Consequently, even if spindles are to be commanded to rotate at the same rpm, it is required that the command be changed over a plurality of times. As a result, operability is poor when controlling a number of spindles.
Accordingly, an object of the present invention is to provide a numerical control apparatus in which control of the rotation of a plurality of spindles is performed efficient fashion and at high speed, and in which it is possible to readily deal with a change in the number of spindles.